There are several desirable reasons for having a service that can determine the position of a mobile radio operating within a cellular telephone system. For example, such a positioning service could be used for locating emergency callers (911) or children positioned within a cellular system. Alternatively, such a positioning service could be used for locating vehicles as part of a dispatching or fleet monitoring system. Also, cellular system operators could use such a positioning service in order to customize service parameters based on an accurate knowledge of mobile telephone location. Such customization could include, for example, providing lower cost services for limited mobility customers. A positioning service would also be of use in locating stolen cellular phones and for investigating fraudulent use of cellular services.
Methods for radio position determination make use of techniques for measuring the propagation delay of a radio signal, which is assumed to travel in a straight line from a transmitter to a receiver at the speed of light. A radio delay measurement in combination with an angle measurement provided by a directive antenna is the fundamental principle of radar location. Radar location is frequently augmented by use of a transponder in the mobile vehicle, rather than relying entirely on the signal reflected by the mobile vehicle.
Alternatively, a so-called trilateration system may be used for locating a mobile radio. In a trilateration system, multiple time delay measurements are made using multiple transmitters and/or receivers. The Loran system is an example of a location system which transmits a series of coded pulses from base stations at known and fixed locations to a mobile receiver. The mobile receiver compares the times of arrival of signals from the different transmitters to determine hyperbolic lines of positions. Similarly, the Global Positioning System (GPS) provides transmission from a set of 24 earth orbiting satellites. Mobile receivers can determine their position by using knowledge of the satellites' locations and the time delay differences between signals received from four or more satellites.
From the above examples, it can be seen that radio position location systems can be divided into two types, those which allow a mobile user to determine its own position, such as GPS, and those which allow another party to determine the position of a mobile transponder such as radar systems. The system disclosed in the present application includes elements of both types, but primarily of the second type, where the fixed portion of a radio system wishes to determine the location of a mobile radio unit positioned within the system. Except in the case of passive radar, such systems generally require the mobile radio unit to transmit a radio signal.
U.S. Pat. No. 5,126,748, entitled "Dual Satellite Navigation Method and System," discloses a method of radio location where the mobile terminal both transmits and receives signals, thereby allowing round trip timing measurements defining circular lines of position to be performed using fewer transmitter sites than required for the Loran and GPS systems in which the mobile terminals contain only receiving capability. In other systems, the mobile terminal may contain only a transmitter and the remaining system elements perform direction finding or multiple receptions of the signal from different locations to determine the position. An example of this is the SARSAT system for locating downed aircraft. In this system, the downed aircraft transmits a signal on the international distress frequency 121.5 MHz (and 273 MHz). An earth orbiting satellite relays the signal to an earth terminal. As the satellite passes overhead, the change in Doppler shift can be detected and a line of position can be determined. Multiple overhead passes by the same or similar satellites can determine a set of lines of position, the intersection of which determines the location of the downed aircraft.
It has long been known that direct sequence spread spectrum signals have useful properties for ranging and position location. Some of the earliest spread spectrum anti-jamming military communications systems also included an accurate ranging capability. GPS is, of course, based on the use of direct sequence spread spectrum waveforms. In GPS, a user's receiver determines its position in four dimensional space-time by making time difference measurements on the signals being received from four or more satellites in view. The satellites are positioned in inclined, 12 hour orbits and arranged so that most of the time in most places, enough satellites will be in view with adequate geometry to permit accurate position calculations. The GPS system informs navigation terminals of current satellite ephemeris information which is required for position calculations.
The Telecommunications Industry Association (TIA) in association with the Electronic Industry Association (EIA) has developed and published an Interim Standard entitled "Mobile Station-Base Station Compatability Standard for Dual-Mode Wideband Spread Spectrum Cellular System," and referred to as TIA/EIA/IS-95-A, May, 1995 (hereafter "the IS-95 standard.") The IS-95 standard supports a code division multiple access (CDMA) cellular system which synchronizes the transmissions of all cells in a cellular system using the GPS satellite downlink signals to update rubidium clocks. Thus, in the IS-95 CDMA system, timing is transferred from the GPS system directly to the cellular system.
The IS-95 CDMA system can determine the location of a mobile station in three dimensional space-time (time plus two dimensional positioning) provided that the mobile receiver is able to receive and track the pilot signals of three neighboring base stations and is provided with accurate location information of the base stations. Likewise, if three IS-95 base stations are able to make timing measurements of a mobile's signal, the system can determine the location of the mobile station. The IS-95 CDMA system implements the universal frequency reuse principal, wherein all sectors and all cells in the system normally operate on the same frequency. This universal frequency reuse principal is central to CDMA's achievement of high system capacity. However, the implementation of the universal frequency reuse prinicpal in a CDMA system can make locating a mobile station problematic in those instances where a mobile station comes close to a base station. In such instances, it may become difficult to achieve an adequate SNR when trying to receive the neighboring base stations. More particularly, transmissions from the neighboring base stations will be jammed by the close-by base station --a classic near/far problem.
A power control system, as described in patents, is necessary to solve the near/far problem for the mobile to base station communication link. As the mobile comes close to one base station, it reduces its transmitter power accordingly so as to achieve a just adequate Eb/No at the closest base station. This results in a lower Eb/No at the neighboring base stations, perhaps making it difficult to receive the mobile's signal at these locations. Thus, as a result of the power control system, neighboring base stations will typically have difficulty measuring mobile signal timing when a mobile unit is positioned near a close-by base station.
In the IS-95 CDMA system, the processing gain is nominally 21 dB. This is simply the ratio of the chip rate (1.2288 MHz) to the maximum data rate (9600 bps). At a point equidistant between two base stations, the transmitter power needed for both base stations is about the same. The resulting SNR at both base stations of the received mobile signal will likely be more than adequate to obtain good timing measurements. However, when the mobile station moves to a point closer to one base station than another, the transmitter power will be reduced. This will lower the received Eb/No at the further away base station. The measurement SNR can be raised by integrating over a longer time interval than a single bit time, effectively increasing the processing gain. For example, if the signal were to be integrated over one code repetition or 32768 chips, the SNR is improved by 24 dB compared to the SNR at 9600 bps because the processing gain becomes 45 dB (10*log 32768). If a 5 dB SNR is needed for good time tracking, then the signal at the far base station can be 30 dB weaker than the close base station. This SNR or better can be achieved in about 90% of the cell area, assuming 4th power propagation. Thus, in 90% of the system's coverage area, the base stations will be typically be capable of time difference measurements in support of positioning, provided that good base station geometry is available to obtain accurate positioning. The 10% of the cell area where time difference measurements between base stations is not available (with the above specified integration time) corresponds to the center of the cell area out to approximately 30% of the maximum cell radius. Thus, for base stations separated by 4 miles (2 mile cell radius) the radius of the area where positioning cannot be done with the above bandwidth assumptions is about 1000 meters.
It should be noted that there are limitations as to the time of integration that might be employed due to Doppler considerations. For example, if a mobile is traveling at 60 mph on a line between two base stations, the differential Doppler is about 2.times.10-7. This amounts to about 170 Hz in the 800 MHz cellular band. This is sufficient Doppler to make integration over 32768 chips somewhat difficult. Thus, the above estimates should be taken as best case.
The basic method of mobile station receive only positioning is described above. In this method, the mobile must receive three or more cell pilot signals from three or more nearby base stations and calculate time differences of arrival of the pilot signals. These arrival time differences allow hyperbolic lines of position to be determined, with the mobile terminal's position being where these hyperbolic lines intersect. However, for the reasons explained above, when the mobile is too close to a base station to obtain an adequate SNR on the two farther away cells, the required signal arrival time differences cannot be easily measured, and therefore some other method must be utilized to determine the position of the mobile station.
It is therefore an object of the present invention to provide a mobile radio positioning system, wherein the position of the mobile radio may determined if the mobile radio is positioned close-by to the closest base station.
These and other objects and advantages of the invention will become more fully apparent from the description and claims which follow or may be learned by the practice of the invention.